isl: Document the 3D block extent of isl_format

src/isl/isl.h

@@ -571,6 +571,13 @@ struct isl_channel_layout {
    uint8_t bits; /**< Size in bits */
 };
 
+/**
+ *   Each format has 3D block extent (width, height, depth). The block extent
+ *   of compressed formats is that of the format's compression block. For
+ *   example, the block extent of ISL_FORMAT_ETC2_RGB8 is (w=4, h=4, d=1).
+ *   The block extent of uncompressed pixel formats, such as
+ *   ISL_FORMAT_R8G8B8A8_UNORM, is is (w=1, h=1, d=1).
+ */
 struct isl_format_layout {
    enum isl_format format;